The present invention relates to a method for handling freight containers. In particular it is a method for determining a desired position on a given stacking target where a container should be placed.
A vast amount of freight is shipped in standard shipping containers. At each point of transfer from one transport means to another, for example in ports and harbours, there is a tremendous number of containers that must be unloaded, transferred to a temporary stack, and later loaded on to another ship or to another form of transport.
In a place for handling containers, such as a container terminal, containers are usually stacked in substantially rectangular groups of stacks. Containers are usually arranged with long axes ordered parallel in substantially parallel rows. The rows are laid out to provide a clearance between containers in each row, and with a clearance between each row of containers. Space in such container yards is at a premium and so the clearance between container rows and containers is required to be minimal and stacks consisting of typically 4-8 containers on top of each other have to be created. High density stacking is also necessary to avoid long transport distances within a container terminal.
The high running cost of ships requires that containers be moved between ship and stack as rapidly as possible so that a ship may be unloaded, loaded and turned around in the shortest possible time. To achieve the shortest unloading and loading times container handling equipment has to be partly and/or preferably completely automated in normal operation.
The technical demands of handling containers are great. The tare weight of containers is usually consistent, but the gross weight varies considerably. The width of shipping containers is standardised at 8 ft, but the height varies between from 8 and 9.5 ft. The most common standard lengths are 20 ft and 40 ft long. The 40 ft container is very common today and even longer containers up to 53 ft long are also in use. Thus the size of containers varies as well as the gross weight. The size of the part of a container that is load bearing, a corner casting, is the same size and area for all sizes of container. With the increase in average length of containers handled it becomes more difficult and more important to place containers accurately in a specified place, known as a stacking target, so as not to create unsafe stacking conditions in the container terminal.
A retrieval problem may occur when a stack of containers is placed so that it leans towards a lower stack of containers. A horizontal clearance is required for vertical access between two rows of containers. If the horizontal clearance between rows is insufficient, it may not be possible for an automatic lifting device either to sense or to access a container at the top of a lower stack.
A container may be handled by a crane, a crane moving on rails, a self-propelled container handling apparatus, or a lift or winch of any type all of which are referred to herein as a crane. Each crane has a lifting device usually incorporating a spreader of some kind that directly contacts a container, to grip it, lift it, lower it and release it. In this description the term spreader is used to denote a part of a lifting device that is in direct contact with a container. Spreaders are normally designed to handle more than one size of container, typically 20-40 ft or 20-40-45 ft long containers.
A problem of safe high density stacking of containers is to keep each stack of containers stacked on a ground slot within predefined limits of the ideal position of the ground slot and a sufficient, but minimum distance, away from other stacks. For safe operation it is also necessary to continuously verify that the criteria are met.
Insufficient horizontal clearance for access to a container on a stack can have more than one possible origin. The first container in the stack, the container in the ground slot, as the position on the ground is called, may have been incorrectly placed relative to an intended location. A container correctly located in a ground slot may subsequently have been moved, by accident for example. Also, one or more containers placed on top of a container in a ground slot may have been misplaced with respect to the container beneath it or later have been moved by accident or, for example, misplaced due to the effect of wind. It is also possible that another container or stack may be placed too close to a container or stack which is otherwise previously correctly positioned. Another factor is that the ground surface may not be sufficiently horizontal. An additional factor is that one or more containers in a stack may be damaged or of poor quality, such that upper and lower surfaces of the container are not parallel, causing a stack to lean.
Another problem is that of unstable stacks. Stacks that are, or become, unstable can have the same origins and factors as above, as well as a possibility that the overlap of the load bearing part of a containers structure may be too small. The load bearing part of a container is a corner casting fitted in each corner of a container. When containers are stacked on top of each other it is the corner castings that bear the load. If the overlap of the corner castings of containers stacked on each other is too small the stack may be unstable.
Within the general requirement of handling containers efficiently there exists a technical problem in determining accurately the correct position where a container should be stacked before it is realistically possible to land a container correctly on a predetermined position. It is also desirable to verify that the stack supporting the stacking target is safe to land a container on and that the correct position can be reached with a safe path that does not interfere with other containers.
It is known from SE 9401120-2 and JP 8 165 086 and some existing installations that attempts have been made to overcome part of the problems by mechanical means, for example, by equipping the container spreader with two so called stacking guides. The guides connect to the bottom of a container to be landed and guide it to line up with the top container beneath it.
The stacking guides however have many disadvantages including that they:
add weight and cost to the spreader,
include a lot of moving parts, sensors etc. which are subject to considerable forces during transfer of container, which makes the stacking guide a maintenance intensive equipment that also reduces the overall availability of the crane,
the guides must have some clearance to the handled container resulting in a position error between the two containers. There is no means of controlling that this does not result in an accumulated error for a whole stack,
require space outside the peripheral dimensions of the container itself, which adds to the space required between stacks and thereby reduces stacking density,
a stacking guide can not assist in positioning the bottom container and can not correct for systematic errors caused either by ground conditions or containers not having parallel upper and lower surfaces,
if the weight of the top container is low it may be displaced by the guides and,
the operation of the stacking guide adds additional time to each landing of a container.
It is also known from GB 0656 868 to base an automatic landing of one container on another by the use of a sensing means to detect the relative position of an upper edge of the target container with respect to the lifting device/container to be landed. This method, however, does not provide any means for determining the ground position of the stack, the accuracy of the complete stack or for avoiding interference from adjacent stacks.
In addition to horizontal errors of placement in a direction parallel with or perpendicular to the line of the row and the long axis of a container when placing a container on a stacking target, a container may also be skewed. A skew is defined here as an angular displacement of the long axis of the container with respect to the line of the row.
In the unpublished Swedish patent application 9803341-8 a system describes one method to determine the precise horizontal location of a ground slot for a container. The system includes the use of one or more horizontal reference markers fixed to the ground adjacent to, and a predetermined distance away from, a ground slot. The horizontal reference markers are designed such that they may be detected by sensors on a travelling part of a moveable crane, and the distance between the crane and horizontal reference marker measured by the sensors. In this way, the horizontal position of a ground slot relative to a travelling part of a crane may be accurately measured and the distance to ground slot, or a container in a ground slot thereby determined.
However determining the position of a stacking target as a surface on top of a container, requires in addition, measurements of the vertical position of the top surface. For a safe stack, information is also required about the relative positions of containers in the stack. In the case where the stacking target is a surface on top of a container which is supported by one or more containers, the horizontal position of that stacking target is also required.
It is an object of the invention to provide a method to determine a position for landing a container on a stacking target based on measurement of the horizontal and vertical position of the stacking target as well as a determined position of a ground container in a stack. It is another object of the invention to determine if a vertical envelope of a stacking target is within predetermined limits and represents a safe stacking target. It is another object of the invention to provide a method to determine a desired position on a stacking target relative to the position of the stacking target and the ground container. It is another object of the invention to determine a safe path to the stacking target. It is another object of the invention to determine a safe distance between the stacking target and one or more containers in one or more stacks adjacent to a stacking target. It is another object of the invention to measure the position of a container after landing on a stacking target and thus verify correct placement.
These and other objects are realised by a method according to the invention. The present invention is a method for determining a desired position on a stacking target for a container based on measuring the position of the stacking target and the ground container in the stack.
The main advantage of the present invention is that a container may be landed in a position on a stacking target that ensures sufficient overlap on each of the 4 corner casting to the container below and that it is landed as close to the horizontal position of the ground container as possible, eliminating any accumulated errors.
Another advantage is that a container may be landed on a desired position on a container comprising a stacking target. A stacking target may be any surface such the top surface of a container stacked in a ground slot, or the top surface of the top container of a stack of two or more containers stacked on a ground slot. A ground slot is used here to describe a stacking target on any substantially horizontal surface such as the ground, a support surface constructed on the ground, a ships deck, a deck or floor inside a building or the loading surface of a vehicle. When containers are stacked on top of each other, it is only the corner castings located in each corner of a container that are load bearing, so it is important that stacked containers overlap each other as closely as possible.
The invention also assures that a container may not be automatically landed on a stack unless it is within predefined limits to the ground container. The invention also ensures that attempts to land automatically are not made if any container in any of the adjacent stacks is interfering with stipulated margins around the path of the container to the determined landing position
The benefits of the present invention include that a container placed on desired position on a stacking target overlaps not only the supporting container making up the stacking target, but also overlaps the ground container or is placed in a mid-position of best overlap between the stacking target and the ground container in the same stack. This means that any error or displacement in the overlap of containers placed one on top of the other in the stack is diminished rather than accumulated. This in turn means more accurately stacked stacks, which permits stacks of containers to be densely packed with respect to each other. The method provides that the total deviation of the stack will not be more than 2 times the positioning accuracy of the crane, and may in practice be better than that.
Another advantage of more accurately stacked stacks is that a safe working distance between stacks is more easy to attain and maintain in practice. This ensures that there is sufficient clearance between adjacent stacks of containers so that containers may be handled efficiently and without damage to containers.
Another advantage is that a container may be automatically lowered into a precisely measured target position. This offers the important economic benefit of rapid handling of containers, which is extremely important in maintaining low and competitive freight costs. Another advantage with an economic benefit is that clearances between stacks and rows may be kept to a minimum, reducing the area of ground needed for storing containers.
The present invention is not limited by direction of approach of a crane towards a ground slot, stacks of differing heights or by containers of differing lengths. The present invention is very useful in facilitating trade by making container handling fast and keeping costs low. It is also not restricted to shipping ports and may be applied to handling any containerised freight, such as air freight, and containers on and off trains or trucks.